Move Objective-C ports to the v2 GNUstep ABI.
* Bump the LLVM revision used for GNUstep to 7, the minimum to support
the new ABI.
* GNUstep-back does not work with lld, so mark it to use Gold (BFD LD
doesn't seem able to link Objective-C things).
* Turn off some annoying debug logs in GNUstep back, which generate
several messages per second when you move the mouse. These should
never have been enabled in a release build anyway.
* Downgrade Cenon to 4.0.2. This was the last version to actually work
with GNUstep (the later ones use XCode >= 5 .xib files, which GNUstep
can't parse).
* Update gorm to git head. The current release doesn't work with the
new Objective-C ABI, but -head has the patches to fix it.
* Update PikoPixel and add it to the gnustep-app meta-package.
* Update the three core GNUstep packages to the latest release.
* Update gnumail and pantomime to the latest release and fix a linking
error with the new ABI.
* Update GNUstep FTP to the latest version.
Reviewed by: bapt (previous version)

Update GNUstep core libraries.
Update the default Objective-C compiler.
Fix various build failures in GNUstep ports from newer versions of the GNUstep
core libraries and from changes to GNUstep Make.
Update various ports that use GNUstep and bump the portrevision of the ones
that haven't had a new release.

All supported versions of FreeBSD that install clang have version 3.4.1 or
newer, so remove obsolete checks in compiler.mk and objc.mk
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D8496

Drop the necessity to add ${PORTSDIR} to dependency line
Modify make describe to automatically prepend ${PORTSDIR} if the path for the
port is not absolute
Checked with poudriere, portmaster, portupgrade
PR: 203685
Exp-run by: antoine
Differential Revision: https://reviews.freebsd.org/D3866

Change the way USES is handled:
- Loop over USES twice, once to define all *_ARGS variables and once to
include Uses/*.mk. This allows all Uses/*.mk to examine arguments given
to other USES entries.
- Always define *_ARGS (possibly empty) and replace commas with spaces.
Similar for _USES_POST.
Adjust all Uses/*.mk:
- defined(u_ARGS) becomes !empty(u_ARGS)
- Eliminate helper variables like _*_ARGS=${*_ARGS:C/,/ /g}
- Some Uses/*.mk used ":" as argument separator instead of ",", but no port
used this form
- Uses/cran.mk: remove unused variable VALID_ARGS and USES+=fortran which
has no effect
- Uses/twisted.mk: simplify handling of the case where neither "build" nor
"run" arguments have been specified
PR: 193931
Exp-run by: antoine
Approved by: portmgr (antoine)